1. Field of the Invention
The present invention relates to a novel liquid crystal display (LCD) structure and to an active matrix based on thin film transistors (TFT) and capacitors, especially adapted to producing high resolution, flat-faced display screens. These flat screens can be used as large, colour television screens, as colour computer terminal screens, with direct viewing, or as a black and white, generally small size projecting screen.
2. Description of the Related Art
In such screen or display types, an electronic memory formed from pixels distributed over the entire surface of the screen stores the video signal for the duration of the image. The liquid crystal in contact with each pixel is excited for the duration of one image. Each pixel is constituted by one or two TFT's and one or two capacitors, the liquid crystal forming the dielectric of one of them. Moreover, these screen or display structures are constituted by two glass plates kept spaced from one another and each having precise patterns between which is interposed the liquid crystal.
The attached FIGS. 1 and 2 diagrammatically show an active matrix, colour display screen structure according to the prior art. FIG. 1 is a longitudinal sectional view and FIG. 2 a perspective view of part of the lower plate of the screen without the layer for passivating and orienting the liquid crystal.
In these drawings, references 2 and 4 respectively indicate the lower and upper, transparent support plates for the screen and reference 6 indicates the liquid crystal.
According to the prior art, the lower plate 2 (FIG. 2) supports on its inner face thin film transistors 8, 9 of each pixel. The transparent electrodes 10 of said pixels form the lower plate of the capacitors. The columns 12 and rows 14 of electrodes control the transistors 8 and 9.
Each addressing column 12 is provided with a bend 16 and each plate 10 is provided with a finger 18. The intersections of the row 14 with the column 12 and the bend 16 define drains D and D' of the transistors 8 and 9 and the intersection of the row 14 with the finger 18 defines the source of said transistors. Those parts of the row 14 respectively located between the column 12 and the finger 18 and the bend 16 and the finger 18 constitute the gates of the transistors 8 and 9.
This assembly is covered with a silicon nitride passivating layer 20 and optionally a liquid crystal orientation layer 22 (FIG. 1).
The upper glass plate 4 has on its inner face colour filters 24 separated by a black matrix 26. These filters are alternately red R, green V and blue B.
These colour filters and the black matrix 26 are covered with a transparent, conductive layer 28 serving as the counterelectrode and in particular the upper electrode for the capacitors. The electrodes 10 and 28 are generally made from indium and tin oxide (ITO). The counterelectrode 28 can also be covered with a layer 29 for aligning the liquid crystal molecules.
A screen structure as shown in FIGS. 1 and 2 is in particular described in SID 84 Digest, pp 308-311 by Y. Ugai et al entitled "Diagonal colour LCD addressed by a-Si TFT'S".
On assembling the screen, the two glass plates 2 and 4 must be positioned facing one another, so that the transparent electrodes 10 defining the pixels are perfectly superimposed on the colour filters 24. Following this positioning, the two plates 2 and 4 are sealed at their periphery with the aid of an adhesive joint 30 and liquid crystal filling of the display cell takes place.
Each of the two screen plates 2 and 4 has very precise patterns. Thus, the spacing of the pixels and therefore the colour filters is typically 0.2 mm. Moreover, each pixel is electrically defined by the electrode 10 of the plate 2 supporting the TFT's and optically by the colour filter 24 surrounded by the black matrix formed under the counterelectrode 28.
In order to bring about a maximum reduction in the width 1 of the black matrix between two consecutive filters and therefore increase to the greatest possible extent the transparency of the screen, a superimposing accuracy of the two plates of .+-.3 .mu.m is sought. However, these two glass plates are known to have a different thermal history during their manufacture.
As is known, glass undergoes a more or less random compaction causing geometrical variations which can reach several micrometers, i.e. more than 10 .mu.m, as a function of the nature of the glass and the temperatures used (particularly for layer deposition).
In this case, the precise superimposing of two glass plates having very precise patterns becomes problematical for large sizes.
This problem of the precise positioning of two glass plates occurs in all screen structures in which the filters and pixels are respectively produced on two support plates and in particular in the structure described in EP-A-179 915.
Moreover, when using the process for the production of a display screen having "two masking levels", as described in FR-A-2 533 072, the transistors 8, 9, as well as the addressing rows 14 are constituted by a stack of an amorphous hydrogenated silicon layer 32, a silicon nitride layer 34 and an aluminium layer 36 in photoetched form. In this type of "gates on top" screen, the lower face of the semiconductor 32 is in direct contact with the glass and is not protected against light.
Although illumination generally takes place through the rear or the upper face, as indicated by the arrows F in FIG. 1, the "bulge" of the TFT'S (or lower faces in contact with the substrate) is exposed to ambient observation light. Furthermore, when said light is intense, a photocurrent is produced in the semiconductor leading to leakage currents in the transistors, which may limit the performance characteristics of the screen and in particular reduce its contrast.
In order to obviate this disadvantage, consideration has been given to the production of a black matrix made from black polyimide below the "bulge" of the transistors, as described in FR-A-2 638 880. However, in this type of screen, the problem of the precise positioning of the colour filters and the electrodes 10 of "pixels" is not solved. This problem of positioning two substrates also exists in the smaller screens used for projection purposes.